The present disclosure relates to a tomographic image generation device, a tomographic image generation method and a tomographic image generation program for obtaining a plurality of projection images of a subject by imaging the subject with different radiation source positions, and generating a tomographic image from the projection images.
In recent years, in order to more closely observe an affected part of the body with a radiographic imaging apparatus using radiation, such as x-ray or γ-ray, tomosynthesis imaging has been proposed, in which imaging is performed by applying radiation to the subject from different radiation source positions by moving the radiation source, and a tomographic image where a desired slice plane is emphasized is generated from the thus obtained projection images. In the tomosynthesis imaging, a plurality of projection images are obtained by imaging a subject with different radiation source positions by moving the radiation source in parallel with the radiation detector or along a circular or ellipsoidal arc trajectory depending on characteristics of the imaging apparatus and necessary tomographic images, and the projection images are reconstructed to generate a tomographic image using a back projection method, such as a simple back projection method or a filtered back projection method. Generating such tomographic images for a plurality of slice planes of the subject allows separating structures overlapping with each other in the depth direction along which the slice planes are located. This in turn allows finding a lesion which is difficult to be detected in a two-dimensional image that is obtained with a conventional simple imaging method. It should be noted that the simple imaging refers to an imaging method that obtains a single two-dimensional image, which is a transmission image of the subject, by a single application of radiation to the subject.
On the other hand, the tomosynthesis imaging has a problem of blur of reconstructed tomographic images due to mechanical errors of the imaging apparatus and body motion, etc., of the subject, which is due to differences in time of imaging operations performed with different radiation source positions. When tomographic images are blurred, it is difficult to find a lesion, such as small calcifications which are useful for early detection of in particular, breast cancer when the subject is the breast. For this reason, it is commonly practiced to obtain both tomographic images and a two-dimensional image by performing the tomosynthesis imaging and the simple imaging.
For this purpose, techniques have been proposed to perform both the tomosynthesis imaging and the simple imaging using a radiographic imaging apparatus for imaging a breast (which is called “mammography”) while the breast is kept compressed (see Japanese Unexamined Patent Publication No. 2007-50264 and PCT Japanese National Phase Publication No. 2012-512669, which will hereinafter be referred to as Patent Documents 1 and 2, respectively).
However, performing both the tomosynthesis imaging and the simple imaging leads to increase of the exposure dose of the subject. To address this problem, a technique has been proposed for reconstructing a tomographic image using also a two-dimensional image obtained by simple imaging, in addition to projection images (see Japanese Unexamined Patent Publication No. 2011-87917, which will hereinafter be referred to as Patent Document 3). Since a two-dimensional image allow detection of fine structures of the subject, as mentioned above, reconstructing a tomographic image using also a two-dimensional image allows improving image quality of the tomographic image. This also allows improving image quality of the tomographic image, which in turn allows reducing the amount of radiation during each imaging operation of the tomosynthesis imaging to reduce the total exposure dose of the subject.
When a radiographic image is taken, a scattered ray removing grid (which will hereinafter be simply referred to as “grid”) is used to prevent lowering of the contrast due to scattered rays of the radiation in the subject. On the other hand, the tomosynthesis imaging is performed by applying radiation to the subject from different radiation source positions, and thus the incidence angle of the radiation relative to the radiation detector varies depending on the imaging position. For this reason, if the grid is used during imaging, vignetting occurs due to the radiation being blocked by the grid depending on the radiation source position, resulting in lower amounts of radiation reaching the radiation detector. Therefore the grid is not used during the tomosynthesis imaging.
Further, during the tomosynthesis imaging, imaging operations are performed with moving the radiation source, and it is necessary to perform each imaging operation with an exposure time of the subject to radiation as short as possible to prevent blur of the projection images. However, a shorter exposure time to radiation results in smaller amounts of radiation applied to the subject, and thus smaller amounts of radiation reaching the radiation detector. For this reason, during tomosynthesis imaging, high energy radiation, which is more easily transmitted through an object, is used to increase the amounts of radiation reaching the radiation detector with an exposure dose as small as possible.